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(meteorobs) Limiting Magnitudes (was Re: NWM1999Dec 12/13 Geminids Florida)
>Norman, the report sounds promising for the peak night. But, are you
>pulling my leg with LM=7.3 ??? Did you estimate this by counting stars
>in the IMO fields, or did you actually see a 7.3 star?
>
>The best I have ever done, at the 7000' level on Mauna Kea is 6.6, and I
>have pretty good night vision.
Uh, oh... Before we reopen those "Limiting Magnitude" debates, let me
quote that handy little IMO Web site one more time:
http://www.imodot net/visual/major01.html#sec52
"The limiting magnitude (which is defined as being the magnitude of the
faintest star near the zenith that the observer can detect using slightly
averted naked eye) defines both the condition of the sky's clarity and the
quality of the observer's eyes. Please note, the limiting magnitude is an
observer specific quantity. Do not be surprised if other observers at the
same site obtain different limiting magnitudes to you. This is the rule,
rather than the exception. Only record your own values! There are several
methods for determining the limiting magnitude. We describe one which is
favoured by many meteor observers... [The Star Count Method]"
Norman does not use IMO Star Counts (unlike myself and many other meteor
observers) because he has his own method, which he has been using since
1960 or so... However, my own IMO Star Counts from the 9000' level on
Mauna Kea during this year's Perseids (I didn't about any stopping point
on that road at 7000'!) considerably exceeded the bounds of IMO's Tables,
LM=7.5, for all nights. (Actually during one hazy period, my LM was down
close to 7.5, or even slightly less.)
I can also add that similar results were obtained by a fellow experienced
meteor observer at the same site, also using the IMO method. Note too that
my LMs during last year's LEO peak, which I observed together with Norman
in Florida, hovered just under 7.5! Finally, note that I don't see mag 7.5
stars everywhere I go: I've never managed better than average LM 7.2 for
any meteor session anywhere in rural New England.
Does this mean you'd have seen mag 7 stars under any of these conditions?
Of course not! Everyone's perception (of faint stars as well as meteors)
is quite unique, and the ONLY reasons for recording LMs are to take into
account observer differences, AND differences in local conditions. So as
we should all keep in mind, LM is just a tool for calibrating data: there
is *NO* benefit to the observer in inflating this or any other number! It
is NOT a number that makes any sense to compete over, any more than you
would compete over meteor counts during a shower peak...
(Now hours of Teff, nights observed, and years of experience are things
some people DO choose to compete over - but that's another topic! ;> )
Clear skies,
Lew Gramer
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